The techniques of transmission in systems comprising multiple antennas have many advantages.
In particular, these techniques make it possible to attain an increased transmission capacity by augmenting the spectral efficiency in the transmission channel through the use of space/time codes at sending. These space/time codes can be used especially to distribute the modulated symbols on the different degrees of freedom of the channel.
FIG. 1 provides a more precise illustration of a transmission scheme in a multi-antenna system.
On the sending side, a binary source signal 10 to be sent undergoes a source encoding 11 and then a channel encoding 12. It then goes through a modulation module 13 designed to convert binary elements into complex symbols: such a module thus associates a complex signal, belonging to a constellation (of the QAM type for example), with a group of bits. Then, a space/time encoding 14 of each group of K symbols is performed and these symbols are then sent on nt sending antennas 151 to 15nt in a MIMO type transmission context.
The signal is then conveyed through a transmission channel and then received on the nr receiver antennas 161 to 16nr. Each receiver antenna receives a linear combination of the symbols sent on each of the nt sending antennas. The signal received is first of all decoded in a space/time decoding module 17 applying a processing operation corresponding to a reverse of the space/time encoding applied when sending.
The equalized signal at output of the space/time decoding block 17 is then fed into a demodulation module 18 and undergoes operations of channel decoding 19 and source decoding 20, delivering an estimated binary signal.
Unfortunately, owing to the use of several antennas, the decoding of the received signals is particularly complex.
Indeed, in order to obtain high decoding performance, it is necessary to use an optimal decoding technique, in terms of maximum likelihood decoding in the decoding module 17, for example sphere decoding or Schnorr-Euchner algorithm decoding.
Now, the complexity of these techniques greatly increases depending on the number of antennas and the number of modulation states.
Some techniques set up a time-out mechanism to limit the time taken to decode a given block and declare a block erasure. A specific block retrieval algorithm is then implemented and another decoder is used to replace the defective decoder.
The principle consists in stopping a decoder after a certain degree of complexity and finishing the decoding by another method. This is not an adaptive decoding because the decoder is fixed at the outset, and the stop criterion that may depend on the channel is used subsequently and not initially in order to choose the decoder.
There is therefore a need for a novel, less complex technique for receiving signals in systems implementing several sending and/or receiving antennas.